R600/SI: Assign a register class to the $vaddr operand for MIMG instructions

R600/SI: Assign a register class to the $vaddr operand for MIMG instructions

The previous code declared the operand as unknown:$vaddr, which made
it possible for scalar registers to be used instead of vector registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188425 91177308-0d34-0410-b5e6-96231b3b80d8